The present invention is generally directed to regulating sodium channels in a cell, and more particularly to a method for modulating sodium channel current in a cell by reactive oxygen species (ROS) originating from mitochondria.
Recently, we reported that mutations in glycerol-3-phosphate dehydrogenase 1-like (GPD1-L) protein, a gene associated with Brugada Syndrome and Sudden Infant Death Syndromes (References 1 and 2), cause reduced cardiac sodium channel (Nav1.5) function by modulating pyridine nucleotides (Reference 3). Elevated intracellular NADH results in a rapid decrease in cardiac Na+ current (INa) in cardiomyocytes that is large enough to be clinically significant (Reference 4) and of a magnitude seen in Brugada Syndrome (Reference 5). The effect is identical on heterologously expressed sodium channel in human embryonic kidney (HEK) cells. The immediacy of the NADH effect on reducing INa and the lack of change in mRNA abundance under various experimental conditions suggests that the effect of NADH is post-transcriptional.
NADH modulated Nav1.5 through PKC activation and increased oxidative stress (Reference 3). The finding that the balance of oxidized and reduced NAD(H) regulates INa suggests that the metabolic state of myocytes may influence Nav1.5. NADH is known to oscillate with myocardial ischemia, and mitochondrial injury is associated with increased NADH and ROS levels (References 6 and 7). These changes in NADH could contribute to reduced INa, conduction block, and arrhythmic risk known to exist with ischemia. Moreover, heart failure is associated with increased oxidative stress, reduced NAD+, and increased NADH (References 8-10). The increased NADH level may contribute to the increased oxidative stress and diminished INa in heart failure (References 11 and 12).
Several metabolic pathways are known to produce ROS, including uncoupled nitric oxide synthase (NOS), the NAD(P)H oxidase, xanthine oxidase, and the mitochondrial electron transport chain (ETC). Cardiac oxidation leads to NOS uncoupling and diastolic dysfunction (Reference 13). NAD(P)H oxidases are an important source of superoxide in human atherosclerosis (Reference 14). Xanthine oxidase plays an important role in various forms of ischemic injury and in chronic heart failure (Reference 15). In ischemia/reperfusion injury, the ETC serves as the source of ROS (Reference 16). In chronic heart failure, ROS levels increase (References 17 and 18) and myocardial antioxidant reserve decreases (References 19 and 20). In turn, ROS increases cell death by apoptosis, reduces cellular respiration, induces structural damage to proteins including ion channels, and impairs contractility (Reference 8).